Ammunition is an essential part of the arsenals of the Armed Forces. A vast array of different types of ammunition are currently in use in the Armed Forces. Conventional ammunition refers to ammunition whereby the projectile is held by and partly extends from the cartridge case. Another type of ammunition is termed Cased Telescoped Ammunition (CTA).
In general, CTA is comprised of individual rounds containing a projectile, fitted inside a cartridge case with seals at both ends, held by an internal steel or composite sleeve. The sleeve itself is internal to the CTA cartridge case and is attached to the front seal by threads. Furthermore, it is designed to prevent the projectile from unwanted movement and also to maintain a necessary alignment with the gun tube once the CTA is fully chambered in the gun.
CTA is being developed by the US Army for use in rapid auto-loader small, medium and large caliber systems up 120 mm range. Presently, a 105-mm CTA is being developed for use in the 105-mm Multi-Role Armament Ammunition System (MRAAS). The term CTA therefore comes from the projectile being telescoped back into the cartridge case. Thus, the CTA ammunition resembles a cylindrical article that houses the projectile, sleeve, and energetics (propellant and primer) internally, hence hidden from view. In contrast, a conventional ammunition is discernible by the aft seal, cartridge case and most of the projectile. Similar to CTA, the energetics are stored inside the cartridge case.
A unique benefit can be provided to both conventional and CTA ammunition by translating the projectile in the gun just before the main propellant charge goes off. Translation means that the projectile is moved or moving just before the main propellant charge provides the energy to fire the projectile from the gun. In brief, the translation process is a mechanism whereby the projectile is displaced a small distance forward in the gun before the main propulsion charge ignites.
The translation affords the projectile a number of advantages. One such advantage is that the projectile is set in motion momentarily before the main propulsion charge ignites, thus reducing the recoiling action of the gun and the setback force on the projectile. Consequently, the impulsive stress on the projectile significantly decreases, thereby improving the performance margin by allowing less robust projectiles to survive gun launch. As a result, the projectile can be made lighter using less robust designs. A lighter projectile will have a higher velocity, and for Kinetic Energy (KE) rounds it will enhance its ability to defeat the target.
Moreover, for ammunition with very high propellant density pack and/or large projectile volume to propellant volume space, high differential pressure waves can occur during propellant ignition. These high differential pressure waves can increase the pressure to dangerous levels that may damage the projectile or the gun. Translation may correct this problem by moving the projectile and correcting the density and volume problem. By translating the projectile, the ullage volume increases, thereby reducing the amplitude of the pressure wave.
Typically, the conventional translation process can be accomplished by an energetic means utilizing a secondary propulsion charge or propellant pre-charge as part of the propulsion system. The secondary propulsion charge is set off, generating a sufficient gas pressure to propel the projectile forward. After a short timing delay following the ignition of the secondary propulsion charge, the main propulsion charge is then ignited, resulting in an ensuing ballistic event of the projectile as it continues to travel along the gun tube and out of the gun to target.
While the conventional propulsion translation design provides the translation objective, such a design involving a dual propulsion charge system is usually difficult to achieve and furthermore presents some risks in maintaining the correct timing. If the timing is too long, then the projectile will travel too far down the tube. Consequently, the propellant gases from the main charge will not impart enough velocity on the projectile to defeat its target. Conversely, if the timing is too short, then the secondary and main charge may go off nearly at the same time, thereby creating a large pressure wave that may damage the projectile or gun.
Another disadvantage with the conventional design using the energetic translation method is that the process is irreversible. Once the secondary propulsion charge is ignited, a ballistic event is eventual and committal. In some cases when a pre-firing termination is commanded, this energetic translation method is not an enabling technology.
Thus, there remains an unsatisfied need for an improved design of a translation process or mechanism for use in conventional and CTA ammunition. Preferably, the enhanced translation design should be easy to achieve in field operation and does not present a risk due to the timing factor. Moreover, the enhanced translation design should be reversible to allow the projectile to return to its initial state after translation in an event of a pre-firing termination.
It is a feature of the present invention to provide an improved design method for achieving a translation process for the projectiles of small, medium and large caliber ammunition including both conventional and CTA. The improved method of translation embodied in the present invention utilizes a spring mechanism built of smart material as part of a mechanism to translate the projectiles.
These smart materials are materials that may be trained to change shape at certain temperatures or when electricity is passed through. They are known as shape memory alloys. Exemplary materials are Nitinol (Nickel-Titanium) and CAN (Copper-Aluminum-Nickel). They can be trained to change to a particular shape at a set temperature or applied current and change back to the original shape. The shape change takes place almost instantaneously and with substantial force to accomplish the work needed to translate a projectile or work a mechanism to translate the projectile. In addition, since they can return to the original shape as needed, the projectiles may be moved back to their pre-translated position if needed. Shape memory alloys have been known to be able change shape thousands of times without loss of properties and ability to do work.
The present invention provides numerous other features, among which are the following:
1. A shape-memory alloy, such as Nitinol, is used for the spring mechanism. The shape-memory alloy retains the information of the spring undeflected state even after undergoing a deflection.
2. For translation of a CTA projectile, the spring mechanism is attached between the steel sleeve on the cartridge housing and the CTA projectile.
3. For translation of a conventional projectile, the spring mechanism is attached between the rear of the projectile and the aft seal at the rear of the cartridge. This same mechanism is also applicable to a CTA projectile.
4.The spring is initially compressed. Upon electrically activating the shape-memory spring mechanism, the spring expands to translate the projectile forward for both the conventional and CTA projectiles.
5. In an event that a pre-firing termination is ordered, an electric charge activates the shape-memory spring mechanism to return it to the initial compressed state, thus restoring the position of the CTA projectile to its non-translated state.
6. Upon firing, the high pressure causes the spring mechanism to separate from the projectile and sleeve or aft seal (case base and seal), and travel up the gun tube and be expelled without interference to the projectile.
The improved method of translation of the present invention affords significant advantages over the conventional design in that the translation mechanism is simple and does not require a propulsion charge, which eliminates the potential risks due to incorrect timing. More importantly, the shape-memory material utilized in the improved translation method of the present invention permits a projectile firing to be terminated before a ballistic event.